Unlike RAID systems of old, but in common with other modern storage technologies such as Solaris' ZFS and Linux's btrfs, pools can use disks of different interface technologies—USB, SATA, Serial Attached SCSI—and different, mismatched sizes. New disks can be added to a pool at any time. Pools can also include one or more hot spares: drives allocated to a pool but kept in standby until another disk in the pool fails, at which point they spring into life.

As expected, Storage Spaces will indeed be a feature of both desktop and server editions of the operating system.
If the feature does indeed ship in desktop Windows, it will overnight obsolete a range of SOHO-oriented storage systems; products like Drobo and ReadyNAS will find it hard to survive in a Windows 8 world.